Not exactly "killing" the aristocracy, but if you mean breaking up the big landed estates which were enabled by primogeniture inheritance then you may be thinking of Estate Duty. Estate Duty (Inheritance Tax) was introduced in Britain in 1894 and significantly increased in 1914 as a tax on capital transferred by inheritance. It is paid as a levy on the estate of the deceased, not by the individual recipients. Since real estate such as a house and land may form a major part of the estate, meeting the inheritance tax may require the sale of these assets, thereby removing large landed estates from the deceased’s families and preventing their being passed down. The period after 1914 saw a break up in family estates. The references to loss of families’ great houses in shows like Downton Abbey, and period novels are usually references to loss by forced sale to meet taxes.